There are theories suggesting that the electrons and quarks are not elementary particles, but instead are composite particles of two or more yet undiscovered elementary particles. However experimentally we know the upper limits of the radii of these particles, and they are very small indeed. Any particles "inside" an electron would be confined to a very small volume. By Heisenberg's uncertainty principle, the particles inside would experience large fluctuations of momentum, making the total energy larger than the mass-energy of electron. Therefore the measured mass of the electron should actually be several orders of magnitude larger. This seems like a paradox.
How do composite-electron and composite-quark theories avoid this?